In some lawn mowers, the oil and gasoline is mixed and added straight to the gas tank. In a Briggs and Stratton lawn mower however, the gas and oil each have their own tank. The lawn mower should have the oil changed at least once per year, but ideally after every 50 hours of use. It is also recommended to change the oil filter every time you change the oil. Read the Owners Manual for your specific Briggs and Stratton lawn mower to find out what type of oil is best and how much to use.
Start the Briggs and Stratton lawn mower. Allow it to run until the engine is warm. Turn it off and disconnect the spark plug.
Lay the mower over onto its side. Place a small bucket underneath the oil drain plug. Use a 3/8 inch socket to remove the bolt to turn the oil drain plug counter clockwise. The oil will begin draining into the bucket.
Replace the oil filter. The filter should twist off easily when turned counter clockwise with a pipe wrench. Pour a bit of clean oil on the new filter. Screw the new filter in by hand in a clockwise direction until it is resting upon the filter adapter, then give it another 1/2 turn.
Pour new oil into the oil drain plug. A funnel may come in handy for this step. Use the type of oil best suited for your style of Briggs and Stratton mower. The oil fill plug also can be used as a measuring cup. Fill it to the level indicated in your mowers owners manual.
Replace the drain plug and tighten it with a 3/8 inch socket in the clockwise direction. Reconnect the spark plug and start your Briggs and Stratton lawn mowers engine.